Bacteria Lab

A solid source for bacterial growth is essential if organisms are to be isolated. In the laboratories of Robert Koch, gelatin was first used to achieve bacteria colonies. Agar now serves as a more useful material; it will remain solid until heating and then will solidify as it cools. Before it cools (and solidifies) it can be poured into petri dishes. Using sterile technique minimizes the contamination of the agar before you are ready to inoculate it with bacteria from other sources.

Safety Precautions

STERILE TECHNIQUE must be used at all times.

ASSUME that any bacteria that grows on your plate is harmful and treat as such.

ALWAYS WASH HANDS after you are finished handling plates.


Preparing Your Plates


1. Without opening any of the petri dishes, use a marking pen to divide the dish into four quadrants and label the sections as shown. TIP: We do not label the lids of dishes because they can easily fall off and be mixed up.
2. Holding the test tube with your left hand, raise the lid of the petri dish with your right (raise only enough to pour the agar). Pour the agar into the plate.
3. The agar will solidify shortly. Turn the dishes upside down, the dishes will be stored and incubated upside down. Until the plates are inoculated, they will be stored in the refrigerator.

Inoculating Your Plates

Surface 1 / Any surface in classroom or hall. Use sterile cotton swabs.
Surface 2 / Any surface in classroom or hall. Use sterile cotton swabs.
Hair / Place a small length of your hair in this quadrant
Finger / Touch the circle marked D with your finger. Then wash your hands and rub your finger with alcohol. Place your “disinfected” finger in circle C

Analyzing Colonies
Identifying and categorizing different isolated bacterial colonies based on varied appearance and morphology (form and structure) will permit the selection and transfer of different species from a mixed culture to a sterile medium. When a single bacterial cell is deposited on the surface of a nutrient medium (agar), it begins to divide exponentially. After thousands of cells are formed, a visible mass appears which is called a COLONY. Each species of bacteria will exhibit characteristic colonies.